Composite sandwich panels are widely used in applications which require engineered structural properties and light weight. A prominent example, among many, is the blades of wind turbines used to produce electrical energy. These blades commonly comprise skins of fibrous reinforcements, for example fiberglass fabric, saturated with hardened resin, for example epoxy or polyester. The skins are bonded to cellular core materials, for example balsa wood, plastics foam or composite core materials. In addition to fiber reinforced resins, sandwich panel skins may comprise a wide variety of other stiff materials, for example aluminum, steel or plywood. Examples of composite core materials are disclosed in U.S. Pat. No. 6,740,381 and U.S. Pat. No. 7,393,577 which issued to the assignee of the present invention and the disclosures of which are herein incorporated by reference.